In conventional flotation methods and apparatuses a valuable substance is extracted from a slurry comprising a mixture of solid phase, liquid phase and said substance. Gas is fed into the slurry in a flotation vessel to infuse gas bubbles into the slurry while the slurry is agitated in the flotation vessel for dispersing the gas bubbles into the slurry. When the particles of the valuable substance come to contact with the gas bubbles they are attached to the bubbles and rise upwards to the surface of the slurry to form a foam bed (herein called as a froth phase) above the free surface of the slurry (slurry phase). The froth can then be removed from the vessel by overflow for further processing.
In the known methods and apparatuses there is a common problem of the entrainment of slurry and liquid pockets, i.e. impurities, from the slurry phase into froth phase. These impurities may exit from the flotation vessel together with the froth overflow causing inefficiency in separation.